26 
BULLETIN 98, U. S. DEPARTMENT OF AGRICULTURE. 
sulphur dioxid is below the atmospheric, any leakage of air will be 
into the system and will cause corrosion of the metal by forming 
sulphurous acid. The low pressures required in using sulphur di- 
oxid as a refrigerant in connection with its large specific volume 
makes a large and cumbersome machine necessary. The ratios of 
the volumes of the cylinders necessary for a given capacity of ma- 
chine, taking that of carbon dioxid as one, are approximately as 
follows: Carbon dioxid 1, ammonia 4.4, sulphur dioxid 13. 
If either liquid ammonia, carbon dioxid, or sulphur dioxid is placed 
in a test tube, as shown in figure 10, it will boil under atmospheric 
pressure below the ordi- 
^>~^ 
*"-~ J'S 
nary temperature of the 
surrounding air, and the 
heat of evaporation will 
pass from the surround- 
ing air directly into the 
refrigerant . The air, 
therefore, will be refrig- 
erated or cooled, due to 
the fact that its heat is 
taken up by the boiling 
liquid. In other words, 
the heat required for 
the evaporation of the 
liquid is extracted from 
the air. At the boiling 
point the refrigerant will 
absorb a definite amount 
of heat from the air to 
effect the vaporization 
of a definite amount of 
the liquid, the heat 
being absorbed directly 
through the walls of the 
test tube from the outside air. There will be a circulation of air 
around the test tube, as indicated by the arrows in the figure, due to 
the greater weight of the cooled film of air lying next the surface of 
the tube which flows down and away from the bottom of the vessel, 
allowing the warmer and therefore lighter air to rise and take its 
place at the top. Figure 10 embodies the principle of the direct- 
expansion system of refrigeration explained in detail later. 
If the test tube containing the refrigerant is immersed in a second 
vessel containing a solution of brine, as shown in figure 11, the 
evaporating liquid will absorb heat from the surrounding medium 
just as in the preceding case, but in this case the surrounding medium 
is brine instead of air and the heat required to effect the vapoiization 
Fig. 10. 
